This invention relates generally to the field of optical communications and in particular to a method and apparatus for imparting long period gratings in an optical fiber.
Long-period gratings can be used to provide a phase-matched coupling necessary to transfer power from one mode of an optical fiber to another. (See, e.g., J. N. Blake, B. Y. Kim and H. Shaw, xe2x80x9cFiber-Optic Modal Coupler Using Periodic Gratings,xe2x80x9d Opt. Lett. 11, 177(1986); J. N. Blake, B. Y. Kim, H. E. Egan, and H. J. Shaw, xe2x80x9cAll-Fiber Acusto-Optic Frequency Shifter,xe2x80x9d Opt. Lett. 11, 389(1986); and J. N. Blak, B. Y. Kim, H. E. Egan, and H. J. Shaw, xe2x80x9cAnalysis of Intermodal Coupling in a Two-Mode Fiber with Periodic Microbends,xe2x80x9d Opt. Lett. 12, 281(1987)). This has proven to be especially useful for coupling between a guiding mode and a cladding mode of ordinary transmission fibers, to create wavelength selective loss (See, e.g., M. Tachibana, R. I. Laming, P. R. Morkel and D. N. Payne, xe2x80x9cErbium-Doped Fiber Amplifier with Flattened Gain Spectrum,xe2x80x9d IEEE Phot. Tech. Lett. 3, 118(1991)).
Long-period gratings are typically written onto a fiber by exposing the fiber to the output of a KrF excimer laser (xcex=248 nm) through an amplitude mask (See, e.g., A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, T. Erdogan, and J. E. Sipe, xe2x80x9cLong-Period Fiber Gratings as Band-Rejection Filters,xe2x80x9d J. Lightwave Tech., 14, 58(1996); A. M. Vengsarkar, J. R. Pedrazzani, J. B. Judkins, P. J. Lemaire, N. S. Bergano, and C. R. Davidson, xe2x80x9cLong-Period Fiber-Grating-Based Gain Equalizers,xe2x80x9d Opt. Lett., 21, 336(1996); and J. A. Rodgers, R. J. Jackman, G. M. Whitesides, J. L. Wagener, and A. M. Vengsarkar, xe2x80x9cUsing Microcontact Printing to Generate Amplitude Photomasks on the Surfaces of Optical Fibers: A Method for Producing In-Fiber Gratings,xe2x80x9d Appl. Phys. Lett. 70,7(1997)). A serious drawback of such laser-written gratings however, is that they are permanent. As a result, once written, their strength (and hence the depth of the loss) is no longer adjustable. Additionally, since the gratings tend to be relatively short (xcx9c25 mm), the resultant absorption band or resonance is fairly wide, typically  greater than 10 nm between the points (henceforth known as the xe2x80x9cFWHdBxe2x80x9d points) where the logarithm of the loss is half that at the peak.
As a result of these and other drawbacks to the prior art, a continuing need exists for improved methods and apparatus that produce long-period gratings in optical fibers.
I have developed a method for inducing long period gratings into an optical fiber mechanically, so that the strength of the resultant resonance can be adjusted continuously (and reversibly) from zero. Briefly stated, my method for inducing gratings into an optical fiber involves gently pressing the optical fiber into a ruled mechanical grating with a rubber or other elastic pad such that a series of xe2x80x9cmicrobendsxe2x80x9d are produced.
Advantageously, my method is both convenient and produces superior results. In particular, gratings constructed according to my method can be long (typically 75-100 mm), and exhibit narrow resonances (typically xe2x89xa73 nm between the FWHdB points) suitable for high resolution correction of erbium amplifier gain curves. Furthermore, if the fiber used for my method is overcoated with a special low-index (n less than 1.4) plastic, there is no need to strip off the coating thereby preserving the strength and durability of the fibers while at the same time, producing gratings that are exceptionally simple and inexpensive to fabricate.
Furthermore, my method also allows one to compensate for random variations in the fiber, which otherwise may produce a small but significant scattering in the resonance wavelengths produced by a particular grating period. Finally, a device constructed according to my method is amenable to electro-mechanical control, since the forces and displacements required to induce the gratings are well within the capability of well-known transducers.
Viewed from one aspect, my invention is directed to the method for producing mechanically induced long period gratings from an optical fiber. Viewed from another aspect, my invention is directed to the long period grating mechanically induced into the optical fiber as taught and described herein.